Antenna efficiency at a frequency of excitation is directly related to the effective electrical length, which is related to the signal propagation rate by the well known equation using the speed of light C in free space, wavelength .lambda., and frequency f: EQU .lambda.=C/f
As is known, antenna electrical length should be one wavelength, one half wavelength (a dipole) or one quarter wavelength with a ground plane to minimize all but real antenna impedances. When these characteristics are not met, antenna impedance changes creating standing waves on the antenna and antenna feed (transmission line), increasing the standing wave ratio all producing energy loss and lower radiated energy.
A typical vertical whip antenna (a monopole) possesses an omnidirectional vertically polarized pattern, and such an antenna can be comparatively small at high frequencies, such as UHF. However, at lower frequencies the size becomes problematic, leading to the very long lines and towers used in the LF and MF bands. The long range transmission qualities in the lower frequency bands are advantageous but the antenna, especially a directional array can be too large to have a compact portable transmitter. Even at high frequencies, it may be advantageous to have a physically smaller antenna with the same efficiency and performance as a conventional monopole or dipole antenna.
Over the years different techniques have been tried to create compact antennas with directional characteristics, especially vertical polarization, which has been found to be more efficient (longer range) than horizontal polarization, the reason being the horizontally polarized antennae sustain more ground wave losses.
In terms of directional characteristics, it is recognized that with certain antenna configurations it is possible to negate the magnetic field produced in the antenna in a particular polarization and at the same time increase the electric field, which is normal to the magnetic field. Similarly, it is possible to negate the electric field and at the same time increase the magnetic field.
The equivalence principle is a well known concept in the field of electromagnetic arts stating that two sources producing the same field inside a given region are said to be equivalent, and that equivalence can be shown between electric current sources and corresponding magnetic current sources. This is explained in Section 3-5 of the 1961 reference Time Harmonic Electromagnetic Fields by R. F. Harrington. For the case of a linear dipole antenna element which carries linear electric currents, the equivalent magnetic source is given by a circular azimuthal ring of magnetic current. A solenoid of electric current is one obvious way to create a linear magnetic current. A solenoid of electric current disposed on a toroidal surface is one way of creating the necessary circular azimuthal ring of magnetic current.
The toroidal helical antenna consists of a helical conductor wound on a toroidal form and offers the characteristics of radiating electromagnetic energy in a pattern that is similar to the pattern of an electric dipole antenna with an axis that is normal to the plane of and concentric with the center of the toroidal form. The effective transmission line impedance of the helical conductor retards, relative to free space propagation rate, the propagation of waves from the conductor feed point around the helical structure. The reduced velocity and circular current in the structure makes it possible to construct a toroidal antenna as much as an order of magnitude or more smaller that the size of a corresponding remnant dipole (linear antenna). The toroidal design has low aspect ratio, since the toroidal helical design is physically smaller than the simple resonant dipole structure, but with similar electrical radiation properties. A simple single-phase feed configuration will give a radiation pattern comparable to a 1/2 wavelength dipole, but in a much smaller package.
In that context, U.S. Pat. Nos. 4,622,558 and 4,751,515 discusses certain aspects of toroidal antennas as a technique for creating a compact antenna by replacing the conventional linear antenna with a self resonant structure that produces vertically polarized radiation that will propagate with lower losses when propagating over the earth. For low frequencies, serf-resonant vertical linear antennas are not practical, as noted previously, and the self-resonant structure explained in these patents goes some way to alleviating the problem of a physically unwieldy and electrically inefficient vertical elements at low frequencies.
The aforementioned patents initially discuss a monofilar toroidal helix as a building block for more complex directional antennas. Those antennas may include multiple conducting paths fed with signals whose relative phase is controlled either with external passive circuits or due to specific self resonant characteristics. In a general sense, the patents discuss the use of so called contrawound toroidal windings to provide vertical polarization. The contrawound toroidal windings discussed in these patents are of an unusual design, having only two terminals, as described in the reference Birdsall, C. K., and Everhart, T. E., "Modified Contra-Wound Helix Circuits for High-Power Traveling Wave Tubes", IRE Transactions on Electron Devices, October, 1956, p. 190. The patents point out that the distinctions between the magnetic and electric fields/currents and extrapolates that physically superimposing two monofilar circuits which are contrawound with respect to one another on a toroid a vertically polarized antenna can be created using a two port signal input. The basis for the design is the linear helix, the design equations for which were originally developed by Kandoian & Sichak in 1953 (mentioned the U.S. Pat. No. 4,622,558).
The prior art, such as the aforementioned patents, speaks in terms of elementary toroidal embodiments as elementary building blocks to more complex structures, such as two toroidal structures oriented to simulate contrawound structures. For instance, the aforementioned patent discusses a torus (complex or simple) that is intended to have an integral number of guided wavelengths around the circumference of the circle defined by the minor axis of the torus.
A simple toroidal antenna, one with a monofilar design, responds to both the electric and magnetic field components of the incoming (received) or outputed (transmitted) signals. On the other hand, muitifilar (multiwinding) may have the same pitch sense or different pitch sense in separate windings on separate toroids, allowing providing antenna directionality and control of polarization. One form of helix is in the form of a ring and bridge design, which exhibits some but not all of the qualities of a basic contrawound winding configuration.
As is known, a linear solenoidal coil creates a linear magnetic field along its central axis. The direction of the magnetic field is in accordance with the "right hand rule", whereby if the fingers of a right hand are curled inward towards the palm and pointed in the direction of the circular current flow in the solenoid, then the direction of the magnetic field is the same as that of the thumb when extended parallel to the axis about which the fingers are curled. (See e.g. FIG. 47, infra.) When this rule is applied for solenoid coils wound in a right-hand sense, as in a right-hand screw thread, both the electric current and the resulting magnetic field point in the same direction, but a coil in a left-hand sense, has the electric current and resulting magnetic field point in opposite directions. The magnetic field created by the solenoidal coil is sometimes termed a magnetic current. By combining a right-hand and left-hand coil on the same axis to create a contra-wound coil and feeding the individual coil elements with oppositely directed currents, the net electric current is effectively reduced to zero, while the net magnetic field is doubled from that of the single coil alone.
As is also known, a balanced electrical transmission line fed by a sinusoidal AC source and terminated with a load impedance propagates waves of currents from the source to the load. The waves reflect at the load and propagate back towards the source, and the net current distribution on the transmission line is found from the sum of the incident and reflected wave components and can be characterized as standing waves on the transmission line. (See e.g. FIG. 13, infra.) With a balanced transmission line, the current components in each conductor at any given point along the line are equal in magnitude but opposite in polarity, which is equivalent to the simultaneous propagation of oppositely polarized by equal magnitude waves along the separate conductors. Along a given conductor, the propagation of a positive current in one direction is equivalent to the propagation of a negative current in the opposite direction. The relative phase of the incident and reflected waves depends upon the impedance of the load element, Z.sub.L. For I.sub.0 =incident current signal and I.sub.a =to reflected current signal, with reference to FIG. 13, infra. then the reflection coefficient .rho.i is defined as: ##EQU1## Since the incident and reflected currents travel in opposite directions, the equivalent reflected current, I.sub.1 '=-I.sub.1 gives the magnitude of the reflected current with respect to the direction of the incident current I.sub.0.